A new tridentate pincer phosphine/N-heterocyclic carbene ligand: Palladium complexes, their structures, and catalytic activities

A new imidazolium salt, 1,3-bis(2-diphenylphosphanylethyl)-3H-imidazol-1- ium chloride (2), for the phosphine/N-heterocyclic carbene-based pincer ligand, PC^NHCP, and its palladium complexes were reported. The complex, [Pd-(PC^NHCP)Cl]Cl (4), was prepared by the common route of silver carbene transfer reaction and a novel direct reaction between the ligand precursor, PC^NHCP·HCl and PdCl₂ without the need of a base. Metathesis reactions of 4 with AgBF₄ in acetonitrile produced [Pd(PC^NHCP)(CH₃CN)](BF₄)₂ (5). The same reaction in the presence of excess pyridine gave [Pd(PC ^NHCP)(py)](BF₄)₂ (6). The X-ray structure determination on 4-6 revealed the chiral twisting of the central imidazole rings from the metal coordination plane. In solution, fast interconversion between left- and right-twisted forms occurs. The twisting reflects the weak π-accepting property of the central NHC in PC^NHCP. The uneven extent of twisting among the three complexes further implies the low rotational barrier about the Pd-NHC bond. Related theoretical computations confirm the small rotational energy barrier about the Pd-NHC bond (ca. 4 kcal/mol). Catalytic applications of 4 and 5 have shown that the complexes are modest catalysts in Suzuki coupling. The complexes were active catalysts in Heck coupling reactions with the dicationic complex 5 being more effective than the monocationic complex 4.